Brushless motors such as switched reluctance motors are increasingly demanded because they are inexpensive and simple in structure. Those motors incorporate an encoder for outputting a pulse signal in synchronism with the rotation of the rotor, which is rotated by sequentially switching the current supply phase by counting pulses of a pulse signal of the encoder and detecting the rotation position of the rotor on the basis of the encoder count. Since it is possible to detect the rotation position of the rotor on the basis of the encoder count after starting, motors of this type having an encoder are used as drive sources of various position switching devices in which a position switching control (positioning control) for rotating the rotor to a target position by means of a feedback control system to, for example, switch the position of an automatic transmission of a vehicle is performed (JP-A-2001-271917).
In such a motor incorporating an encoder, a feedback control is performed in such a manner that the rotor is rotated toward a target position by switching the current supply phase on the basis of the encoder count in synchronism with pulses of a pulse signal of the encoder. When the encoder count has reached a target count that is set in accordance with the target position, the feedback control is finished with a determination that the rotor has reached the target position. The rotor is stopped at the target position.
Here, to generate torque for rotating the rotor, the phase of the current supply phase needs to lead the rotation phase of the rotor. The rotor rotation speed can be made high by increasing the phase lead of the current supply phase in such a manner as not to cause loss of synchronization. However, if the rotor rotation speed in a feedback control is increased, because of its inertia the rotor tends to overshoot a target position after the end of the feedback control: it is difficult to stop the rotor correctly at the target position.
Further, in a system that performs a feedback control for rotating the rotor to a target position on the basis of the encoder count, a failure of the feedback control system or an abnormality (e.g., noise, loss of a pulse, or a disconnection of a signal line) in output pulses of the encoder may cause loss of synchronization between the current supply phase (i.e., encoder count) and the rotation phase of the rotor, which may lead to a situation that the rotor cannot be driven normally or is rendered out of control.
One countermeasure is as follows. When a situation that the motor cannot be feedback-controlled normally has occurred, switching is made from the feedback control to a fail-safe control (i.e., open-loop control). The current supply phase of the motor is switched sequentially by supplying a drive signal to a motor drive circuit without feeding back encoder count information. Pulses of the drive signal are counted and the rotor is rotated to a target position on the basis of resulting counts.
However, during the fail-safe control, the rotor rotation angle (i.e., the manipulated variable of a position switching mechanism) is merely estimated on the basis of the count of the drive signal. Therefore, the actual rotor rotation angle may deviate from the estimated value. As a result, the shift position of an automatic transmission of a vehicle may be determined erroneously during the fail-safe control.